Analysis

Trees could take up to four years to return to normal
growth rates in the aftermath of a severe drought, a new study
finds.

With the frequency and severity of droughts
likely to increase with climate change, we might
not be able to rely on forests to absorb as much of our carbon
emissions, the researchers say.

Drought stress

Forests hold almost half
of the carbon found on the Earth's surface, storing it in
their woody trunks and branches.
Studies show that forests are sensitive to droughts,
causing tress stress and limiting how much they can grow and store
carbon.

During the European heatwave in 2003, for example,
tree and plant growth
fell by 30%. That meant the land surface in Europe actually
produced more carbon dioxide than it absorbed that year.

The new study, published in
Science, suggests that it takes longer for trees to recover
after a severe drought than previously thought.

Tree rings

Using data from the International Tree
Ring Data Bank, researchers analysed tree growth at over 1,300
sites across the northern hemisphere countries. The sites are
predominantly in North America and Europe, and oak and pine trees
make up the majority of the species the researchers considered.

Tree rings provide a handy estimate of how quickly a tree has
grown. As a tree grows, it puts on extra layers of wood around its
trunk, creating a new ring each year. The quicker a tree grows, the
bigger the gap between tree rings from one year to the
next.

Analysis of a global survey finds that more than
a third of the world's adults have never heard of climate change.
For some countries, such as South Africa, Bangladesh and Nigeria,
this rises to more than two-thirds of the adult
population.

The study says that education is the "single
strongest predictor" of public awareness of climate change.
Improving basic education and public understanding of climate
change are vital to garner support for climate action, the
researchers add.

Awareness and concern

The new study, published inNature
Climate Change, uses the results of aGallup
World Pollin 2007-08, which collected responses
in 119 countries. This is the largest survey ever conducted on
climate change, the paper's authors tell Carbon Brief, representing
more than 90% of the world's population.

The poll asked people: "How much do you know
about global warming or climate change?" Those who were aware of
the issue were then asked the follow-up question: "How serious a
threat is global warming to you and your family?"

The results show that adults in developed countries
were more likely to say they are aware of climate change. Awareness
rates in much of North America and Europe were well over 90% of
respondents. Japan comes top with 99% of the population aware of
climate change, with the US (98%) and Finland (98%) following
closely behind.

An average of
3.5m square kilometresof land go up in smoke each
year as a result of wildfires. Annual carbon dioxide released in
these infernos can exceed half theemissionsfrom humans burning fossil
fuels.

A new study finds that the number of days
wildfires are likely to burn each year is increasing as global
temperatures rise.

Researchers estimate that between 1979 and 2013,
the wildfire season has lengthened by an average of 19% for more
than a quarter of the Earth's vegetated surface.

Wildfires play an important role in
flammable ecosystems, such as forests, grasslands, savannas,
and shrublands. They can be
managed to disperse plants, clear forests and promote grazing,
or suppressed to protect human lives and property.

Most wildfires are triggered by
humans -as much as 90% in the US, for example -
while natural causes include lightning and
lava. But the weather is thebiggest
driverof how much area that wildfires actually
burn. Temperature, humidity, rainfall and wind speed all play a
role in providing the right conditions for a fire.

A new study, published inNature
Communications, finds that changes in these
different weather variables are conspiring to increase the risks of
wildfires.

Fire season length

Researchers analysed three global weather
datasets to develop a metric for "fire weather season length" - the
number of days per month where conditions create a high fire
danger. They then worked out how the season length had changed
between 1979 and 2013 for vegetated areas across the world. You can
see the results in the graphs below.

Graphs show a) Global average wildfire season
length (expressed as a standardised anomaly), and b) Total global
average area experiencing 'long' wildfire seasons (as a % of global
vegetated area) - both from 1979 to 2013. Source: Jolly et al. (2015)

Throughout human history, large volcanic eruptions
have affected the year-to-year variability of the
Earth's climate and even triggered crop failures and
famines. These events may also have contributed to disease
pandemics and the decline of agriculture-based societies.

In our study published today in the journal Nature,
we used ice-core records to provide a new reconstruction of the
timing of nearly 300 individual volcanic eruptions extending as far
back as the early Roman period. And then we worked out the radiative
forcing of these eruptions - or how they have affected the
energy balance of the Earth.

Summer cooling

When volcanoes erupt, they inject large amounts of
sulphur dioxide into the upper atmosphere. These combine with
oxygen and water to form
sulphate aerosols, which shield the Earth's surface from
incoming solar radiation and cause cooler temperatures for as long
as two years after an eruption.

We derived our reconstruction of past eruptions by
looking for these aerosols in more than 20 individual ice cores
extracted from ice sheets in Greenland and Antarctica.

These new records show that large volcanic eruptions
in the tropics and high latitudes were the dominant drivers of
climate variability, responsible for numerous and widespread summer
cooling extremes during the past 2,500 years.

Our study shows that 15 of the 16 coldest summers
recorded between 500 BC and 1,000 AD followed large volcanic
eruptions - with four of the coldest occurring shortly after the
largest volcanic events.

Our team of 24 scientists from the United States,
United Kingdom, Switzerland, Germany, Denmark, and Sweden verified
the timing of these events with the help of tree ring data. To
align the two types of data, we used a distinctive signature of an
extra-terrestrial cosmic ray event around 774-775 AD, which would
could see in the tree rings and ice cores.

The odds of the UK having a winter as cold as the one
in 2009-10 will drop to less than 1% by the end of the century as
global temperature rise, researchers from the UK Met Office
say.

The new research combines long-term projections of
climate change with the yearly ups and downs of the UK's
notoriously changeable weather.

The results suggest that very cold winters and wet
summers will become less and less likely, but the research says we
should still expect them from time to time.

Cold snap

Climate projections tend to focus on how the typical
weather of each season will change in the future. Yet British
weather is often anything but typical.

Projections of climate change tell us that UK winters
are likely to
get milder. Yet the winter of 2009-10 was the coldest in
England and Wales for over 30 years, with average temperatures
around
2C below the 1971-2000 average.

But there isn't a contradiction at all, they say. And
so they set out to show how a cold winter or a wet summer could
still happen in a much warmer world - even if they are much less
likely.

Weather and climate

The researchers used the 'UK Climate Projections 2009'
(UKCP09 )
- a set of projections of our future climate averaged into 30-year
chunks, from 2010-2039, all the way to 2070-2099. They then
combined them with model projections that are averaged over just a
single year, and thus include more of the year-to-year variability
in our weather.

The resulting graphs, shown below, illustrate how
different aspects of our weather are expected to change under a
moderate emissions scenario. The black lines show observed changes
and the red, blue and yellow lines show three (of many) different
runs of their climate model.

Observed (black lines) and projected (coloured
lines) changes temperature/rainfall for individual seasons under
the
A1B emissions scenario for England and Wales. Data show
difference from the 1961-90 average. Source: Sexton and
Harris (2015)

The results show how even with a clear climate change
trend, we are still likely to be on the receiving end of some highs
and lows of British weather, the researchers say.

Which of the many thousands of papers on climate
change published each year in scientific journals are the most
successful? Which ones have done the most to advance scientists'
understanding, alter the course of climate change research, or
inspire future generations?

On Wednesday, Carbon Brief will reveal the results of
our
analysis into which scientific papers on the topic of climate
change are the most "cited". That means, how many times other
scientists have mentioned them in their own published research.
It's a pretty good measure of how much impact a paper has had in
the science world.

But there are other ways to measure influence. Before
we reveal the figures on the most-cited research, Carbon Brief has
asked climate experts what they think are the most
influential papers.

We asked all the coordinating lead authors, lead
authors and review editors on the last Intergovernmental Panel on
Climate Change (IPCC) report to nominate three papers from any time
in history. This is the exact question we posed:

"What do you consider to be
the three most influential papers in the field of climate
change?"

As you might expect from a broad mix of physical
scientists, economists, social scientists and policy experts, the
nominations spanned a range of topics and historical periods,
capturing some of the great climate pioneers and the very latest
climate economics research.

With eight nominations, a seminal paper by Syukuro
Manabe and Richard. T. Wetherald published in the Journal of
the Atmospheric Sciences in 1967 tops the Carbon Brief poll as
the IPCC scientists' top choice for the most influential climate
change paper of all time.

Entitled, "Thermal Equilibrium of the Atmosphere with
a Given Distribution of Relative Humidity", the work was the first
to represent the fundamental elements of the Earth's climate in a
computer model, and to explore what doubling carbon dioxide (CO2)
would do to global temperature.

Manabe & Wetherald
(1967), Journal of the Atmospheric Sciences

The Manabe & Wetherald paper is considered by many
as a pioneering effort in the field of climate modelling, one that
effectively opened the door to projecting future climate change.
And the value of climate sensitivity is something climate
scientists are
still grappling with today.

Over the past few decades, our Sun has been relatively
active, giving off high levels of the solar radiation that warms
the Earth. However, in recent years this peak activity has tailed
off, prompting scientists to wonder if the Sun is heading into a
period of lower output.

A new study says even if the Sun's activity did drop
off for a while, it wouldn't have much impact on rising global
temperatures. But it could mean a higher chance of a chilly winter
in Europe and the US, the researchers say.

Solar output

The Sun's activity rises and falls on an approximately
11-year cycle, but it can experience longer variations from one
century to another. Over the past 10,000 years, the Sun has hit
around 30 periods of very high or very low activity - called 'grand
maxima' and 'grand minima'.

In 2010, scientists
predicted an 8% chance that we could return to Maunder
Minimum conditions within the next 40 years.

But since that study was published, solar activity has
declined further, and this likelihood has increased to 15 or 20%,
says new research published today in open-access journal Nature
Communications.

In fact, the Sun's output has declined faster than any
time in our 9,300-year record, say the researchers. And so they set
out to analyse what this could mean for global and regional
climate.

Small decrease

The researchers used a climate model to run two
scenarios where solar activity declines to a grand minimum. They
then compared the results with a control scenario where the Sun
continues on its regular cycle.

For all model runs they used the
RCP8.5scenario to account for future climate
change - this is the scenario with the highest greenhouse gas
emissions of those used by the Intergovernmental Panel on Climate
Change (IPCC). Global emissions are currentlytracking just abovethis
scenario.

You can see the modelling results in the maps below.
Overall, a grand solar minimum could see global average temperature
rise trimmed by around 0.12C for the second half of this century,
the researchers say. Larger changes (shown as dark greens and
blues) are seen in some parts of the northern hemisphere.Projected difference in annual average
surface temperature for 2050-99 between RCP8.5 emissions scenario
and a) Solar scenario 1 and b) Solar scenario 2. Areas of blue and
green show regions projected to be cooler because of the solar
minimum. Source: Ineson, S. et al. (2015)

Curbing climate change could be the biggest global
health opportunity of the 21st century. But if we choose not to
act, we could reverse all the progress made by economic development
in the last 50 years towards improving global public health.

These are the conclusions of a new report by the
Lancet Commission out today.

Curbing air pollution, phasing out coal, access to
clean energy worldwide and promoting healthier lifestyles would
have "immediate gains" for human health, says the report.

The authors also call for a global price on carbon and
a scaling-up of adaptation financing.

The Lancet Commission is a body set up to map out the impacts of
climate change on health, and make recommendations to improve
health standards worldwide.

Today's report is a collaboration between European and
Chinese academics across the physical, health, political and social
sciences, economics, energy policy and engineering.

"After only 0.85C warming,
many anticipated threats have already become real-world
impacts."

And if we continue to track the highest emissions
scenarios - taking us to
4C or 5C by the end of the century - the risk of
potentially catastrophic impacts rises even higher, the report
adds.

Changing exposure in over 65s to heatwaves by 2090 for
RCP8.5 (left). Growth in annual heatwave exposure for over 65s with
and without accounting for a growing and ageing population (right).
Source: Lancet Commission report on health and climate change
(2015)

The impacts of climate change on human health are
all-pervading. Small risks can interact to produce
larger-than-expected chances of catastrophic outcomes, the report
notes.

As well as the
direct effects of rising temperatures on heat stress,
floods, drought and other extreme weather, climate change increases
air pollution, alters the spread of disease and raises the risk of
food insecurity, malnutrition, migration and conflict.

Scientists are calling for a rethink in the way we
seek to understand how climate change affects extreme weather.

The latest in so-called attribution studies is to
study each
individual event by itself, looking for how climate change
may have made it stronger or more likely.

But a new paper says the methods used in many of these studies
underestimate the influence of climate change, and suggests a new
approach to identify the "true likelihood of human influence".

Single-event attribution

One of the first studies to attribute a single extreme
weather event to climate change was published just over a decade
ago. Researchers
showed that climate change had doubled the chances of the
record heatwave Europe experienced in 2003.

In the years that followed, many more studies have
aimed to provide answers on how climate change is affecting our
most brutal weather.

But while scientists have been able to attribute
events caused by temperature extremes, linking other extreme events
like storms and heavy rainfall events has proved more difficult,
says a new paper in Nature
Climate Change.

Difference in temperature for 20 July to
20 August 2003 compared to long-term average. Source: Reto Stockli
and Robert Simmon (NASA).

In our chaotic weather system, the complex dynamics of
the atmosphere mean the size and path of a storm or heavy rainfall
event has a large element of chance, the authors say. This can make
it tricky to identify where climate change fits in.

But rather than analysing the wind patterns that bring
a storm to an area, scientists should be looking at how the impact
of that storm has been boosted by temperature changes - known
as thermodynamic effects.

Temperature increases mean more moisture evaporates
into the atmosphere and more ice melts into our warming oceans,
raising their levels. These are changes that scientists can be
confident of, the authors say, and so should be the basis for
attribution studies - rather than looking at changes to circulation
patterns in the atmosphere.

A global project that's been instrumental in shaping
scientists' understanding of how the oceans affect our climate
celebrated its tenth birthday recently.

A new paper published in
Science looks back at 10 years of the
RAPID project, which has been keeping tabs on how heat moves
around in the Atlantic Ocean since 2004.

Over its short lifetime, the project has thrown up a
few surprises. Parts of the Atlantic circulation seem to have
slowed down, though whether that's down to human activity remains
to be seen.

Carbon Brief talks to one of RAPID's founding
scientists, Prof
Harry Bryden from the National Oceanography Centre in
Southampton, about the project's past and future.

Global heat transport

Above about 1,000 metres in the North Atlantic, warm
water flows northwards from the equator towards the poles,
releasing heat as it goes. The water cools and sinks at high
latitudes, returning southwards towards the equator at much deeper
depths.

This is known as the Atlantic Meridional Overturning Circulation
(AMOC) and forms part of a
global ocean conveyor belt that transports heat all around
the world.

The Gulf Stream - another component of the AMOC - is
driven by the wind. Heat released to the atmosphere as the warm
Gulf Stream moves northward gives northwest
Europe its mild climate.

All components of the AMOC together transport about 18
million cubic metres of water per second - equivalent to a hundred
times the flow from the Amazon river. The heat carried with it
means North Atlantic sea surface temperature is about
5C warmer than in the North Pacific at similar
latitudes.